do you think doing this would be mean?

[Nat]

I think it all depends on how recently you broke up with him, let's say it was less than four months ago, Your ex may not understand why you broke up with him and be pretty sad, and seeing something like that is (believe me) absolutely no fun. But if it was longer than four months then he should try to move on and you should be able to use whatever profile picture you like.
Basically it all depends on the circumstances you are in, if you don't mind hurting him: go for it, If you know it wont hurt him: go for it, If it will hurt him: don't do it. But at the end of the day it is your choice. And also I agree with LP7's answer